The present invention relates to an apparatus which senses an acoustic emission resulting from a micro fracture of an object, such as cracking, its growth, or fatigue, deterioration of the object, to detect its micro fracture.
The acoustic emission phenomenon is well-known in which when an object fractures microscopically, a part of its strain energy is emitted as ultrasonic waves, and this phenomenon has been utilized for detecting an abnormality of a material, equipment and the like by monitoring the acoustic emission (hereinafter referred to as AE) signal. But conventional detecting methods of this kind are intended primarily for measuring parameters in the time domain, that is, the intensity, the position of generation, the frequency of generation and the duration of the AE signal, and they encounter difficulty in removing external noises, such as a vibration sound and electrical noise, and call for a special circuit for discriminating between the noise and the AE signal.
There has also been proposed a method which, noting the difference in frequency spectrum between the AE signal and the external noise, discriminates between them on the basis of the difference between their attenuation factors by some filters of different frequency characteristics. This method does not aim at the frequency analysis and the discriminating filter cannot be designed unless the spectrum of the AE signal is preknown. Recently a method has been proposed which decides an abnormality by comparing the spectrum pattern of the AE signal and a reference spectrum pattern. Since the spectrum pattern of the AE signal undergoes substantial variations with the quality, the structure, the fracture process and so on of the object under inspection, it is very difficult to determine the reference spectrum pattern; therefore, there has been a strong demand for an apparatus which permits the real-time frequency analysis. In practice, however, there has not been proposed the processor that performs the real-time frequency analysis of the AE signal which has frequency components over as wide a frequency band as several ten KHz to 1 MHz.